1. Field of the Invention
This invention relates to a color separation prism system for use in a color television camera, and more particularly to improvements in a color separation prism for use in a color television camera provided with a bias light source.
2. Description of the Prior Art
It has been known in the art to use a bias light means in a color separation prism system in a color television camera in order to eliminate residual images formed on the pick up tubes in the camera. The bias light is independent of the light from the object televised and is introduced to uniformly illuminate the face of the tubes to eliminate the residual images thereon. The art of using a bias light in a color television camera is disclosed, for instance, in U.S. Pat. No. 3,824,004.
On the other hand, various kinds of ENG (Electronic News Gathering) cameras for the purpose of gathering news have recently been developed. This kind of camera is desired to be compact and have a lens of a diameter as large as F:1.4 so as to be easily handled and operable in comparatively dark places. In order to make the camera as compact as possible, it is desirable to use glass of high refractive index on the order of nd=1.7 for the glass material of the prisms. By using a glass with a high refractive index such as nd=1.7, the volume of the color separation prism system can be reduced to 1/4 to 1/5 that of the conventional color separation prism system using BK7 glass having a refractive index of nd=1.5168 when the diameter of the lens is made as large as F:1.4. Furthermore, by using glass of high refractive index to make the volume of the color separation prism system small, it is made possible to use a zoom lens system having a comparatively short back focal length, which facilitates the design and manufacture of the zoom lens system of a small size and high performance.
The color separation prism system using the glass of high refractive index which has the above described advantages, however, is disadvantageous in that when a bias light is introduced therein a total reflection occurs in the prism system and the three color image pick-up tubes for red, green and blue are not all equally exposed to the bias light.